1. Brief Description of the Invention
This invention relates generally to apparatus and methods for monitoring and/or control of industrial processes using diode laser-based sensors, in particular monitoring and/or control of combustion processes.
2. Related Art
Monitoring and control of combustion processes requires the use of sensors to provide information on gas species composition and temperature. Information on the combustion state of the process from the sensor can be assessed either by an operator or by automated process control system to determine the necessary operating parameter adjustments needed. Effective control of the process parameters provides an optimization scheme that can impact the energy efficiency, product quality, production rate, and pollutant reduction.
For combustion processes operating with hydrocarbon fuels, knowledge of the O2 and CO concentration is essential to determine whether the combustion space is reducing or oxidizing. Typically, combustion space monitoring is conducted either by using extractive sampling probes or by in-situ probes. Extractive sampling uses water or gas cooled probe inserted into the process stream. The sample is extracted by drawing the process gas through the probe using a pump followed by a chiller system to remove the water vapor before reaching the gas analyzer. This technology suffers from probe and sample line plugging and other maintenance issues such as corrosion when applied to harsh process environments as encountered in glass melting tanks, steel processing furnaces, chemical processing systems, etc. In addition to maintenance issues, long sampling lines reaching up to several hundred feet are not uncommon resulting in sampling delay times that can range form seconds to several minutes. For control and optimization on dynamic processes (time varying O2 and CO concentration) such as electric arc furnaces or secondary aluminum melters, a real-time measure of the combustion state is needed to adequately control the process. Sensor systems that introduce long delay times reduce the controllability and observability of the system.
In-situ monitoring of the combustion space reduces the effect of delay time; however, true in-situ probes for monitoring both O2 and CO are not commercially available. Instead hybrid in-situ systems are used such as AMETEK model WDG-HPII that continuously extracts the gas sample to the analyzer mounted on the process. This system analyzes the O2 concentration with a zirconium oxide probe and the combustibles (CO, H2, CxHy) with a catalytic detector before returning the gas stream back to the process. Despite the elimination of the sampling line, response times are still on the order of 10's of seconds and maintenance issues persist on high particle density applications, since the gas is drawn through the instrument.
The drawbacks associated with conventional monitoring technologies can be avoided by using non-intrusive optical techniques based on measuring absorbed radiation from a source that propagates through a medium. In particular, the emergence of tunable diode lasers in the near-infrared provide a source of radiation that can access absorption transitions of important chemical species such as CO, O2, H2O, CO2, etc. providing an alternative analytical measurement technique that has been demonstrated in both laboratory and industrial settings. The measurement is conducted by launching a beam of radiation across the process to a receiver that monitors the modulated radiation intensity. By ramping the injection current to the device the laser can be rapidly tuned across a resonance absorption transition of the targeted specie to record an absorption spectrum that contains both the baseline and the absorption line feature. The Beer-Lambert relation describes the resulting absorption of the laser radiation along the measurement path for a single species given byIv=Iv,oe[−S(T)g(v−vo)Nl]  (1)where Iv is the laser intensity at frequency v measured after the beam has propagated across a path l with N absorbing molecules per volume. The incident laser intensity is Iv,o and is referred to as the reference. The amount of laser radiation attenuated is determined by the temperature dependent linestrength S(T) and the lineshape function g(v−vo). Inversion of Eq. 1 relates the number density N to the measured laser intensities and known linestrength and pathlength given by                     N        =                              1                                          S                ⁡                                  (                  T                  )                                            ⁢              l                                ⁢                      ∫                                          ln                ⁡                                  (                                                            I                      vo                                                              I                      v                                                        )                                            ⁢                              ⅆ                v                                                                        (        2        )            The rapid tunability of the diode laser (reported values up to 1000 Hz by Allen, M. G., “Diode Laser Absorption Sensors for Gas-Dynamic and Combustion Flows”, Measurement Science and Technology, Vol. 9, pg. 545–562, 1998) allows signal averaging of several hundreds of spectra over a short time interval (<1 sec). The fast time response of the technique provides essentially real-time process monitoring suitable for dynamic monitoring and control.
In monitoring applications on industrial processes, multiple species or multiple absorption line detection is often required to obtain sufficient information on the state of the process for monitoring and control purposes. However, the current tuning range for standard diode lasers, for example, distributed feedback or vertical cavity surface emitting lasers (VCSEL's), are limited to only ˜1–3 cm−1. Therefore, in only few selected examples can multiple species (or absorption lines) be monitored using a single laser. For example, Thomson et al. (“Laser Based Optical Measurements of Electric Arc Furnace Off-Gas For Pollution Control and Energy Efficiency”, Innovative Technologies for Steel and Other Materials, Met Soc., The Conference of Metallurgists, Toronto, August 2000) demonstrated CO and H2O monitoring at 1577.96 and 1578.1 nm, respectively, using a single diode laser and jump scan technique to monitor both species.
To expand the number of monitored species, systems have been designed utilizing fiber optic components to integrate multiple lasers together. The compatibility of near-infrared (“NIR”) lasers with fiber optic components provides a means to multiplex systems together using several diode lasers to access the targeted wavelength regions. A demonstration of the multiplexing capability is shown by Furlong et al., “Diode-Laser Sensors for Real-Time Control of Temperature and H2O in Pulsed Combustion Systems,” 34th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, AIAA-98-3949, 1998, where multiple H2O absorption lines are monitored using two lasers at 1.392 and 1.343 μm. Here fiber optic couplers were used to combine the output of each laser into a single fiber to transport the radiation to the process. The ability to launch the radiation across the process from a single fiber minimizes optical access requirements and reduces the complexity of the optical system. However, at the receiver side, the beam is composed of multiple wavelengths that must be demultiplexed using dispersive elements to separate the radiation and direct it to individual detectors. Alternatively, time domain multiplexing can be used, where the laser scans are conducted at different phases providing near simultaneous detection of the species at the receiver. (Allen, M. “Diode laser absorption sensors for gas-dynamics and combustion flows”, Meas. Sci. Technol. 9 (1998) pp. 545–562.) Regardless of the means used, multiplexing into a single fiber with the criteria of maintaining single-mode transmission, minimum energy loses, and insensitivity to mechanical stress such as bending and vibration requires that the input wavelengths are reasonably close. The constraint placed on utilizing a single fiber for transporting multiple wavelengths while maintaining single mode transmission is described by                     D        <                              2.4            ⁢            λ                                π            ⁢                                                            n                  o                  2                                -                                  n                  1                  2                                                                                        (        3        )            where D is the maximum core fiber diameter, λ is the wavelength, n0 is the refractive index of the fiber core and n1 is the refractive index of the fiber cladding. If the core diameter, D, is larger than the right hand side (“RHS”) of the inequality, then the fiber can carry multiple modes. Rearranging equation 3 to solve for λ defines the cutoff wavelength, i.e., for single mode transmission, the wavelength must be greater than the cutoff wavelength for a given set of fiber parameters.
For combustion monitoring applications to monitor and/or control the degree of reducing or oxidizing atmosphere, the important combustion species are CO and O2. In the near infrared CO absorption is described by the second overtone (3,0) band near 1560 nm whereas O2 absorption is monitored from the b-X (0,0) band near 760 nm. In this case, the large difference in wavelengths prohibits combining the output of each laser into a single fiber while minimizing energy loses and maintaining single-mode transmission. To maintain single-mode transmission, a fiber diameter of 5 μm is required for 760 nm and 9 μm for 1560 nm.
In addition to fiber incompatibility over the broad wavelength range, any transparent optics used in the system such as beam expanders, collimators, focusing lenses, windows, and the like, preferably have an antireflective coating to minimize reflections propagating through the optical train. Without suppressing the resulting reflections from the optical surfaces, interference fringe patterns (etalons) on the recorded absorbance spectrum are produced, thus degrading the quality of the measurement. In the case of O2 and CO detection an antireflective coating functioning over the broad wavelength range spanning 800 nm is required. The maximum off-the-shelf broadband antireflective coating available can approach ˜500 nm in the NIR over the range of 1050–1600 nm, which is not sufficient for use in an O2 and CO monitoring application. Instead a double vee anti-reflective (“AR”) coating that has a minimum reflectivity over a narrow bandwidth at 760 and 1560 nm can be used. The wavelength dependence on AR coated optics and fiber optic diameter constraints have led to previous multiple species detection schemes to use separate optical access for monitoring CO and O2, as disclosed by Frontini et al. of Finmeccanica S.p.A. (U.S. Pat. No. 5,832,842) where a single laser plus detector system is used per species requiring multiple line-of-sight access points on the process. Similarly, commercially available diode laser systems for monitoring O2 and CO are presently manufactured by Norsk Electroptics (Norway) and Altoptronic (Sweden). However, in both cases each species is monitored using a dedicated beam launch and receiver unit for each species. Multiple species monitoring would require not only additional dedicated systems, but also additional line-of-sight optical access ports on the process. This approach is both costly and cumbersome, limiting the application to only cases where the additional cost can be justified. It is therefore desirable to interface the optical system to the process with minimum components and minimum optical access points.
A demonstration of the integration of a broad range of wavelengths into a single system was shown by Ebert et al. for O2 (760 nm), H2O (812 nm), and CH4 (1650 nm) monitoring on a 1 GW powerplant (Ebert, V., et al., “Simultaneous Diode Laser-Based In situ Detection of Multiple Species and Temperature in Gas-Fired Power Plant”, Proceedings of the Twenty-Eighth Symposium (International) on Combustion, The Combustion Institute, Vol. 28, pp. 423–430, 2000) and O2 (760 nm) and H2O (812 nm) monitoring on a 20 MW waste incinerator (Ebert, V., et al., “Simultaneous Laser-based In situ Detection of Oxygen and Water in a Waste Incinerator For Active Combustion Control Purposes”, Proceedings of the Twenty-Seventh Symposium (International) on Combustion, The Combustion Institute, Vol. 27, pp. 1301–1308, 1998). In this case, all lasers were mounted close to the process with associated optics for collimating and overlapping the beams. On the receiver side, the multiple wavelength-transmitted beam was separated onto multiple detectors using narrowband interference filters and focusing mirrors. In this case, the measurements were conducted by direct absorption using lasers that were not fiber optically coupled, thus having a specific polarization allowing the use of non-anti-reflective coated Brewster angled windows. Though the system incorporates a broad wavelength range, issues related to placement of the sensitive lasers and associated electronics near the harsh combustion process and the complexity of the optical arrangement to collimate and combine the beams are significant drawbacks. For industrial applications, the lack of robustness is not acceptable for continues day-to-day operation.